The present embodiments relate to a method for compensating for eddy current fields in magnetic resonance images acquired using a magnetic resonance apparatus.
Artifacts induced by eddy currents represent a problem in a number of magnetic resonance techniques, including imaging and spectroscopic methods. An example of this is diffusion imaging (e.g., diffusion-weighted echo planar imaging (EPI)), in which high gradient amplitudes (e.g., diffusion gradients) are combined with a great sensitivity to dynamic field interferences (e.g., approximately 10 Hz per pixel in the phase-encoding direction). Distortions having an appearance that is dependent both on the amplitude of the gradients (e.g., diffusion weighting) and on the direction of the gradients are observed. In the case of spectroscopic magnetic resonance techniques, eddy-current-induced artifacts, which may be caused by strong localization gradients, cause, for example, undesirable changes in line shape or width. Turbo spin echo (TSE) imaging is also cited in this connection, since in acquisitions of joint images, for example, a volume of interest that is located at a distance from the isocenter may be used, such that eddy current fields that may result in artifacts in the images may be present.
It is known to provide a magnetic resonance apparatus with actively shielded gradient coils that significantly reduce the penetration of the gradient fields into conductive structures (e.g., a cryostat of a magnet). Also known are pre-distortion (“pre-emphasis”) methods, using which remaining zeroth- and first-order eddy current components may be suppressed further by modifying the control of a reference oscillator and/or of the gradient coil. Examples of pre-emphasis methods are disclosed in U.S. Pat. No. 7,002,343 B2 and U.S. Pat. No. 5,451,877, for example. For these methods, it is known to measure the time-dynamic response of the eddy current fields for specific parameters, for example, during tune-up (e.g., installation of the magnetic resonance apparatus) and/or during maintenance. The eddy current fields may be described as a linear system, such that the eddy current fields may be determined from the pre-measurement how the reference oscillator and the gradient coil are to be driven so that the actual time responses correspond to the target time responses in an optimized approximation. In practice, the pre-distortions may be added to the target time response using mathematical methods, and the modified time response is used as an amended control parameter for control purposes.
While these measures are adequate for many imaging methods in order to achieve a diagnostic image quality, in the case of very sensitive magnetic resonance techniques (e.g., the diffusion-weighted EPI imaging or localized spectroscopy), serious artifacts continue to occur in the data in spite of these measures.
Further improvements are proposed, a basic distinction being made between “active” and “passive” measures. Within the scope of the present embodiments, the descriptor “active” may refer to a measure, in which there is an intervention into the physical image acquisition process, where with “passive” measures, a post-processing of the acquired magnetic resonance data (e.g., image processing) is performed.
One known active method that may be employed in addition to a pre-emphasis method is described, for example, in U.S. Pat. No. 5,864,233. Prior knowledge of the zeroth- and first-order eddy current field global components remaining (e.g., for a specific image acquisition protocol) is used for active global compensation. In order to compensate for the zeroth-order global component, a dynamic change in the phase and/or frequency of a reference oscillator provided, for example, at the receiving device may be compensated for, while first-order global components are to be eliminated using an offset in the control of the gradients. An active compensation of low-order global effects is described.
Passive measures have also been described, for example, in German patent applications DE 10 2010 013 605.0 and DE 10 2010 001 577.6. The disadvantage of these methods is due to the purely passive character: information that has already been lost during the acquisition process may not be reconstructed.
With regard to the active methods (e.g., by aligning the imaging gradients for the first order and adjusting the frequency and/or phase of the reference oscillator for the zeroth order), in the case of a magnetic resonance apparatus with actively shielded gradient coils and implemented pre-emphasis methods, the higher-order effects constitute the dominant component among the occurring artifacts (e.g., compare in this regard German patent application DE 10 2010 001 577.6). The active correction known, for example, from U.S. Pat. No. 5,864,233 is not able to compensate for the higher-order effects, while the cited passive methods have the disadvantage that information may be lost during the image acquisition process.